Gold immersion solution for treating silver and method of applying same



United States Patent GOLD IMMERSION SOLUTION FOR TREATING SILVER AND METHOD OF APPLYING SAME Frank X. McNaliy, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a can poration of Pennsylvania No Drawing. Application April 30, 1953 Serial No. 352,305

2 Claims. (Cl. 117-130) This invention relates to the coating of metals by immersion without external electrolytic action, and, more particularly, to the coating of silver with gold in this manner.

It is common practice in the electrical industry to coat copper members with silver to improve the electrical conducivity thereof. The oxides resulting from the corrosion of copper act as an insulator which is undesirable at electrical contact points, whereas the oxides of silver are good conductors. A difiiculty encountered with silver, however, is that it is easily tarnished and corroded by the sulphides and moisture generally encountered in the atmosphere, and particularly in and around industrial installations. Gold is highly resistant to corrosion and it would be desirable to provide a thin protective film of gold over silver if this could be accomplished economically and conveniently. A thin film of about 0.00001 inch thickness will prevent corrosion-of silver surfaces without impairing the low contact resistance.

An object of this invention is to provide a solution for plating silver rapidly and economically with a gold coating by immersion only.

A further object of this invention is to provide a method for depositing gold on silver surfaces by immersing the silver surfaces in a solution comprising essentially an aqueous acid solution of gold chloride.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description.

I have discovered a novel solution for coating or plating silver rapidly and economically with a gold film by immersion only, such solution comprising an aqueous acid solution of gold chlride. Such solutions comprise gold chloride, certain mineral acids to maintain a pH of 1.0 to 0.3 and, if desired, a buffer. It has been found that a concentration of approximately one gram of gold chloride per liter of solution will give good results, although the solution will deposit gold until exhaustion of the gold therefrom. Solutions having a higher content of gold chloride may be prepared and will also give good results.

The acids used in the plating solution of this invention may be either hydrochloric or sulfuric acid, or mixtures thereof. A sufiicient amount of acid is admixed with the aqueous gold chloride solution to maintain the pH thereof at about 1.0 to 0.3. The acid may be added to the water either before or after adding the gold chloride.

If desired, a buffer may be admixed with the plating solution in order to aid in the control of the deposition rate of the gold and also to stabilize the solution. Examples of suitable buffers are acetic acid, sodium citrate, and potassium citrate or mixtures thereof. The individual amounts may be varied depending upon con- 2,836,515 I Patented May27, 1958 "ice ditions, but, generally, 20 grams of approximately 99.5% glacial acetic acid per liter of solution or 30 grams of sodium citrate or potassium citrate per liter of solution have given good results.

The temperature of the solution is not critical. During immersion of the members to be deposited with the gold coating. the fastest rate of coating occurs when the solution is maintained at a temperature of from 190 F. to boiling. At lower temperatures, it is desirable, but not necessary, to keep the solution agitated by mechanical stirring to get good results. An excellent immersion gold plate has been obtained on silver from a solution at 130 F. The time of immersion in the solution may be varied according to the thickness of the coating desired, but from one to five minutes is sufficient time to deposit an adequate protective coating for elecrical purposes.

When depositing a gold coating on a member that has been silver plated in any conventional manner, it is desiderable, but not necessary, that the silver plated memher be immersed in the gold plating solution soon after it has been removed from the silver plating solution. If the silver plated member is allowed to become soiled, a non-uniform and splotchy gold coating may result. However, this silver may be cleaned and will then be coated properly with the gold.

Solid silver members may be coated with gold by means of the plating solution of this invention, but the silver member should be given a bright mechanical polish or an acid dip, and preferably both, immediately before immersion in the gold solution.

The following is an example of the formulation of an aqueous acid gold chloride plating solution and its application in depositing a gold coating on a member in accordance with this invention.

Example One gram of gold chloride AuCl .2H O, 30 grams of trisodium citrate Na C H O .l1H O, and milliliters of 37% hydrochloric acid were admixed with enough Water to make one liter of solution and the resulting solution was then heated to a boiling temperature, and continuously agitated by mechanical means. A copper coil, to be used to carry high frequency currents, was silver plated in a conventional manner, rinsed in water, and was then immediately immersed into the agitated gold chloride solution for a period of time of three minutes. A uniform gold coating was deposited on the silver plated coil. The color of the gold coating was practically identical with solid gold. The gold coating was quite adherent. Further, the gold deposit was also formed in inaccessible recesses that otherwise could not be readily gold plated by electrolytic means.

Since certain obvious changes may be made in the above procedure and dilferent embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the method of depositing a gold coating on a metallic member having a silver surface, without the use of externally applied electrical curernt, the steps comprising immersing the member in a solution comprising a mixture of gold chloride and Water, with an acid present in an amount sufiicient to maintain the pH of the solution at about 1.0 to 0.3, the acid being selected from the group consisting of hydrochloric and sulfuric acids, and removing the member from the solution after a period of time of at least one minute whereby gold is immersion plated on the silver surface.

2. The method of claim 1 wherein the solution comprises a bufier selected from at least one of the group 7 FOREIGN PATENTS consisting of sodium citrate, potassium citrate, and acetic 15,641 Great Britain Nov. 15, 1887 7 acid, the buffer acting to stablhze the Solution and control the deposition rate of the gold. v OTHER REFERENCES References Cited in the file ofythis patent 7 Frary. Transactions of American Electrochemical,

7 Society, vol. 23 (1913),.pp. 27-30. Copy in Division 56. UNITED STA ES PATENTS Saintpierre, Compt, Rend., vol. 54 (1862). 2,501,737 Porter et a1. Mar. 28, 1950 V r 2,560,979 Pessel July 17, 1951' 10 

1. IN THE METHOD OF DEPOSITING A GOLD COATING ON A METALLIC MEMBER HAVING A SILVER SURFACE, WITHOUT THE USE OF EXTERNALLY APPLIED ELECTRICAL CURRENT, THE STEPS COMPRISING IMMERSING THE MEMBER IN A SOLUTION COMPRISING A MIXTURE OF GOLD CHLORIDE AND WATER, WITH AN ACID PRESENT IN AN AMOUNT SUFFICIENT TO MAINTAIN THE PH OF THE SOLUTION AT ABOUT 1.0 TO 0.3, THE ACID BEING SELECTED FROM THE GROUP CONSITING OF HYDROCHLORIC AND SULFURIC ACIDS, AND REMOVING THE MEMBER FROM THE SOLUTION AFTER A PERIOD OF TIME OF AT LEAST ONE MINUTE WHEREBY GOLD IS IMMERSION PLATED ON THE SILVER SURFACE. 